1. Field of the Invention
The present invention relates to a double-balanced mixer circuit used for mixing signals of different frequencies.
2. Description of Related Art
In a portable telephone, for example, a local oscillation frequency signal and an intermediate frequency signal are mixed in a mixer circuit to generate a radio frequency signal which is to be transmitted. Because mixing of signals having different frequencies is based on a non-linear characteristic, outputs from the mixer circuit include, in addition to the radio frequency signal obtained by mixing, the individual local oscillation frequency signal which is not mixed and the individual intermediate frequency signal which is not mixed. In order to suppress the individual local oscillation frequency signal and the individual intermediate frequency signal, a double-balanced mixer circuit is commonly used for the mixer circuit.
FIG. 1 shows a circuit diagram of a conventional double-balanced mixer circuit. The double-balanced mixer circuit comprises a differential amplifier circuit DFA1 which outputs a signal LO.sub.1 of the same phase as the local oscillation frequency signal LO and a signal #LO.sub.1 obtained by an 180.degree. phase shift on the local oscillation frequency signal LO upon input of the local oscillation frequency signal LO (frequency LO), a differential amplifier circuit DFA2 which outputs a signal IF.sub.1 of the same phase as the intermediate frequency signal IF (frequency IF) and a signal #IF.sub.1 obtained by 180.degree. phase shift on the intermediate frequency signal IF upon input of the intermediate frequency signal IF, and an analog multiplier circuit ALG which mixes the local oscillation frequency signal LO and the intermediate frequency signal IF upon input of the signals LO.sub.1 and #LO.sub.1 which are output from the differential amplifier circuit DFA1 and the signals IF.sub.1 and #IF.sub.1 which are output from the differential amplifier circuit DFA2.
In the differential amplifier circuit DFA1, a power supply V.sub.DD is grounded via ann series circuit of an FET (field effect transistor) 2 and a resistor R.sub.1, and is grounded via a series circuit of a resistor R.sub.2, an FET 3 and an FET 4. In parallel with the series circuit of the resistor R.sub.2 and the FET 3, a series circuit of a resistor R.sub.3 and a FET 5 is connected. The power supply V.sub.DD is further grounded via a series circuit of an FET 6 and a resistor R.sub.4, with a capacitor C.sub.1 being connected in parallel with the resistor R.sub.4. The junction of the FET 2 and the resistor R.sub.1 is connected to one terminal of the capacitor C.sub.2 and to the gate of the FET 3. The local oscillation frequency signal LO is fed to the other terminal of the capacitor C.sub.2. The gate of the FET 5 is connected to the junction of the FET 6, the resistor R.sub.4 and the capacitor C.sub.1. The signal LO.sub.1 of the same phase as the local oscillation frequency signal LO is output from the junction of the resistor R.sub.2 and the FET 3, and the signal #LO.sub.1 of the phase being shifted from that of the local oscillation frequency signal LO by 180.degree. is output from the junction of the resistor R.sub.3 and the FET 5.
The differential amplifier circuit DFA2 is made in a similar constitution as that of the differential amplifier circuit DFA1, and identical components are assigned the same numerals. In the differential amplifier circuit DFA2, the intermediate frequency signal IF is supplied to other terminal of the capacitor C.sub.2. The signal IF.sub.1 of the same phase as the intermediate frequency signal IF is output from the junction of the resistor R.sub.2 and the FET 3, and a signal #IF.sub.1 of the phase being shifted from that of the intermediate frequency signal IF by 180.degree. is output from the junction of the resistor R.sub.3 and the FET 5.
In the analog multiplier circuit ALG, a power source V.sub.CC is connected to one terminal of a current source 10 via a series circuit of a resistor R.sub.10 (R.sub.11), a transistor Q.sub.1 (Q.sub.4) and a transistor Q.sub.5 (Q.sub.6), with another terminal of the current source 10 being grounded. The junction of the resistor R.sub.10 (R.sub.11) and the transistor Q.sub.1 (Q.sub.4) is connected to the junction of the transistor Q.sub.4 (Q.sub.1) and the transistor Q.sub.6 (Q.sub.5) via the transistor Q.sub.3 (Q.sub.2). The bases of the transistors Q.sub.2 and Q.sub.3 are connected to each other and the bases of the transistors Q.sub.1 and Q.sub.4 are connected to each other. The radio frequency signal RF generated by mixing the local oscillation frequency signal LO and the intermediate frequency signal IF is output from the junction of the resistor R.sub.11, the transistor Q.sub.2 and the transistor Q.sub.4.
On the other hand, the signal LO.sub.1 (#LO.sub.1) from the differential amplifier circuit DFA1 is inputted to the bases of the transistors Q.sub.2 and Q.sub.3 (Q.sub.1 and Q.sub.4), and the signal IF.sub.1 (#IF.sub.1) from the differential amplifier circuit DFA2 is inputted to the bases of the transistor Q.sub.6 (Q.sub.5).
The operation of the double-balanced mixer circuit will now be described below.
When the local oscillation frequency signal LO is inputted to the differential amplifier circuit DFA1, the signal LO.sub.1 having the same phase as the local oscillation frequency LO and the signal #LO.sub.1 having a phase 180.degree. shifted from the local oscillation frequency LO are inputted from the differential amplifier circuit DFA1 to the analog multiplier circuit ALG. When the intermediate frequency signal IF is inputted to the differential amplifier circuit DFA2, the signal IF.sub.1 having the same phase as the intermediate frequency signal IF and the signal #IF.sub.1 having a phase 180.degree. shifted from the intermediate frequency signal IF are inputted from the differential amplifier circuit DFA2 to the analog multiplier circuit ALG. Then the analog multiplier circuit ALG outputs the radio frequency signals RF having frequencies of LO+IF and LO-IF generated by mixing the local oscillation frequency signal LO and the intermediate frequency signal IF.
The signal LO.sub.1 and the signal #LO.sub.1 cancel each other, and the signal IF.sub.1 and the signal #IF.sub.1 cancel each other, so that the signals LO.sub.1, #LO.sub.1, IF.sub.1 and #IF.sub.1 are not output individually, thereby improving the S/N ratio of the radio frequency signal RF.
The phase of a signal to be mixed can also be shifted by changing the path length. And the required change in the path length decreases as the frequency becomes higher. Therefore the method of changing the path length is employed in applications with frequencies 10 GHz or higher, in consideration of the degree of circuit integration.
Such a circuit is also employed in which phase inverter circuits are employed for the differential amplifier circuits DFA1 and DFA2 for phase shift. FIG. 2 shows a circuit diagram of a phase inverter circuit. The power supply V.sub.DD is grounded via a circuit comprising a parallel connection of a series circuit of a resistor R.sub.20 and a resistor R.sub.21, a series circuit of a resistor R.sub.22, an FET 10 and a resistor R.sub.23, a series circuit of an FET 11 and a resistor R.sub.24, and a series circuit of an FET 12 and a resistor R.sub.25.
The junction of the resistors R.sub.20 and R.sub.21 is connected to one terminal of a capacitor C.sub.10 and to the gate of the FET 10, while the local oscillation frequency signal LO or the intermediate frequency signal IF is fed to the other terminal of the capacitor C.sub.10.
The junction of the resistor R.sub.22 and the FET 10 is connected to the gate of the FET 12, and the junction of the FET 10 and the resistor R.sub.23 is connected to the gate of the FET 11. The signal LO.sub.1 having the same phase as that of the local oscillation frequency signal LO or the signal IF.sub.1 having the same phase as that of the intermediate frequency signal IF is output from the junction of the FET 12 and the resistor R.sub.25 via the capacitor C.sub.11. The signal #LO.sub.1 having a phase being shifted by 180.degree. from the local oscillation frequency signal LO or the signal #IF.sub.1 having a phase being shifted by 180.degree. from the intermediate frequency signal IF is output from the junction of the FET 11 and the resistor R.sub.24 via the capacitor C.sub.12.
Also in the case where this phase inverter circuit is used, similarly to the case where the differential amplifier circuit is employed, the signal LO.sub.1 of the same phase as the local oscillation frequency signal LO and the signal #LO.sub.1 obtained by 180.degree. phase shift from the local oscillation frequency signal LO are obtained from the phase inverter circuit upon input of the local oscillation frequency signal LO, and the signal IF.sub.1 of the same phase as that of the intermediate frequency signal IF and the signal #IF.sub.1 obtained by 180.degree. phase shift from the intermediate frequency signal IF are obtained from the phase inverter circuit upon input of the intermediate frequency signal IF.
The conventional double-balanced mixer circuit as described above is capable of good double-balanced mixing operation with less signal attenuation. However, because the conventional double-balanced mixer circuit comprises the differential amplifier circuits or the phase inverter circuit employing a number of FETs and the analog multiplier circuit employing a number of transistors, it consumes a considerable amount of power, and is therefore not suitable for the application in a portable telephone which has a limitation in the power consumption. Because the differential amplifier circuit, the phase inverter circuit and the analog multiplier circuit employ cascaded FETs and transistors, a significant voltage drop occurs and it is difficult to drive the portable telephone by a low-voltage power source, for example a 3 V power source, which is required in the portable telephone.
Also because a portable telephone uses the radio frequency signal RF having a frequency of 1.9 GHz, a GaAs MESFET (Metal Semiconductor FET) which has excellent high-frequency characteristics is more suitable than transistors based on Si. However, a GaAs MESFET is likely to depart from the linear small signal operation region when a signal having voltage amplitude greater than a certain level (for example, 0.1 to 0.2 V or higher) is inputted to the gate thereof, making it impossible to obtain a linear output. Because the local oscillation frequency signal and the intermediate frequency signal commonly used in a portable telephone have voltage amplitude of 0.1 V or higher, in a differential amplifier circuit made by using GaAs MESFETs as described above, a gate of the GaAs MESFET receives an input voltage higher than the level which gives a linear output, and results in such a problem as the distortion of the phase-shifted signals.